The present invention relates generally to position detection and more particularly, to a system and method for determining the position of a drive member using a positioning disk adapted to be rotated with that drive member.
For the more precise infusion of parenteral fluids into a patient, infusion pumps have been developed. These pumps have allowed greater control over the infusion rate and have been beneficial in the administration of parenteral fluids in many ways. However, there is a continuing desire for improvement in the accuracy and precision of these pumps over a wide range of flow rates.
One type of infusion pump is a linear peristaltic pump. In this pump, a drive motor rotates a shaft to cause cams to move pinching fingers into and out of contact with a fluid conduit causing the sequential occlusion in a wave-like motion of the conduit. This sequential occlusion forces the parenteral fluid from a reservoir into the patient at a programmed flow rate. The cams are disposed along the camshaft so that adjacent cam lobes project at different angular positions relative to the camshaft. The occluding fingers follow the respective cam lobes as the motor rotates. The drive motor is typically a stepper motor having a selected number of motor steps per complete rotation; for example, two-hundred steps. In general, the stepper motor rotates in a sequential, incremental, step-by-step manner over a complete rotation, the complete rotation defining a pump cycle.
Each incremental movement or step of the motor causes a corresponding incremental movement of the cams and fingers that results in a discrete volume of fluid being pumped. This discrete volume may be referred to as a "step volume". Because of the various mechanical parts involved, their inter-relationships and the linear configuration of the pumping mechanism, an inherent characteristic of linear peristaltic pumps is that step volumes deviate from each other. Furthermore, the mechanical configuration of the linear peristaltic pump results in reverse flow in the flexible conduit at some point or points in the pumping cycle and the step volumes at those points is actually negative.
The particular volumetric quantity pumped by each movement of a pumping mechanism can be measured by means such as gravimetric measurement and can then be stored for later reference. A stepper motor provides a convenient reference for pump mechanism position and movement in that defined increments of movement or "steps" exist with stepper motors. Because the stepper motor is typically rigidly mounted to the camshaft, a step of the motor always results in a movement increment of the camshaft, the cams, and the pumping fingers. Thus, determining a position of the camshaft enables one to determine the position of the motor driving the camshaft. In cases where step volumes are considered in the control of the pump, the position of the drive member must be known so that the step volume can be considered. Such an application may exist where flow uniformity is to be increased. Therefore, more precise motor position information is required.
Upon start up of an infusion pump the drive member position is typically unknown by the pump processor. Because of this condition, a pump that considers step volumes to increase flow uniformity cannot begin its control of the motor position for this purpose and must let the pump run until the position can be determined. The processor must first determine at which step the motor is located. It is therefore desirable for the processor to be able to determine the motor drive member position quickly so that the quantity of fluid pumped before that position is determined is not excessive.
In one previous system to determine motor position, a positioning disk is attached to the drive motor shaft and has a plurality of concentric circular tracks of different radii, each track formed with openings of different angular widths. Associated with each track is an optical sensor, such as a light emitting diode coupled with a photo-detector, to detect the angular openings. The openings are configured on the plurality of tracks to correspond to predetermined binary codes representative of a particular motor position. The openings are sensed during rotation of the encoder disk and the particular motor position determined. However, this system incorporates multiple sensors with their attendant multiple output signals resulting in increased complexity and expense and requiring more complex software to decipher the many data inputs to derive a particular motor position.
One less complicated system utilizes a single flag-type indicator positioned on the motor drive shaft for indicating position. Some linear peristaltic systems use this flag to indicate the end of the normal pump cycle and the beginning of the speed-up cycle. While hardware and software costs are reduced with such a system, it can take almost one complete rotation of the motor to determine the drive member position when the motor start up position is just after the flag. Allowing this amount of time and movement to pass before being able to determine the drive member position can be undesirable especially where negative flow steps are involved.
Another consideration in using position disks or flags is the accuracy required in physically mounting them on the cam shaft. Likewise, where multiple cams are used, they must be accurately positioned on the camshaft in relation to each other and to the position disk. Where all of these parts are separate, each must be individually aligned resulting in increased complexity and expense.
Hence, those skilled in the art have found it desirable to provide more precise information pertaining to the position of a drive member while at the same time, lowering hardware and software costs. The system should be capable of quickly and efficiently determining the drive member rotational position without excessive pumping before that position is determined and without the use of relatively expensive, complex hardware or software. It has also been recognized as desirable to provide an infusion pumping system that alleviates the necessity of individually aligning multiple pumping and position detection. components. The present invention fulfills these needs and others.